Aqueous HCl as Mobile Phase Constituent

[Ron Klug]

I'm using 4 mM aqueous HCl (pH = 2.5) as the primary (95%) mobile phase constituent for a polar C18 separation. Is this liable to corrode the pumping system? I'd be interested in anyone's personal experience (good or bad) using HCl in this manner. Thanks!

[Uwe Neue]

Chlorides in general and HCl in particular are supposed to attack stainless steel. I have seen people use HCl in so-called inert systems, but not in standard steel systems.

[HW Mueller]

We have been through this many times, extensively!
But: Take a piece of stainless (preferably one section shined) and dump it into your HCl. Let it stand for some weeks, occasionally observing what happens (and please tell us about the results). I know that at higher HCl conc., stainless turns black very fast ("normal" iron, a nut, turned red brown). At a concentration of 0.15M Cl- at neutral or higher pH didn´t do anything in months to the stainless used here. I have used PBS (0.15M phosphate, 0.15M NaCl, pH ~ 7.2) for years without any problems (don´t leave the stuff on there longer than needed, though).

[Bill Tindall]

Is there a reason that you are not using phosphoric acid which we all know to be safe to use in LC systems over extended periodes of time?

[Ron Klug]

Is there a reason that you are not using phosphoric acid which we all know to be safe to use in LC systems over extended periodes of time?

I've been thunking my forehead every time I think about it now. I'm too far down the road to just switch to phosphoric at this point...unless corrosion is a real issue. But you are right, I know of no good reason why phosphoric acid would not work in my application.

[Uwe Neue]

Hmmh, not a good spot to be in...

If the method will need to be run on other systems, I would follow the general advice given by the engineers who design such systems and rework the method. No question.

If the method will run only on my own system, and if it is a lot of work to redevelop and requalify the method, I might take the risk and run it on my machine. 4 mM? Maybe that is not enough to do any serious harm...

[Ron Klug]

We have been through this many times, extensively!
But: Take a piece of stainless (preferably one section shined) and dump it into your HCl. Let it stand for some weeks, occasionally observing what happens (and please tell us about the results). I know that at higher HCl conc., stainless turns black very fast ("normal" iron, a nut, turned red brown). At a concentration of 0.15M Cl- at neutral or higher pH didn´t do anything in months to the stainless used here. I have used PBS (0.15M phosphate, 0.15M NaCl, pH ~ 7.2) for years without any problems (don´t leave the stuff on there longer than needed, though).

I left a shiny column fitting in 10 ml of the aqueous mobile phase per your suggestion. After 3 weeks no noticeable physical changes. I also analyzed the solution for iron and detected none (<1 ppm). I thank you and others for your helpful comments.

[Chris Pohl]

Ron,

As Hans says, we'd been through this discussion many times and I haven't chimed in as the discussion regarding compatibility of stainless steel with chloride containing eluents always seems to go nowhere. But given your post, I think it's necessary to add a couple of clarifying points. First off, the test you described is inadequate for determining what happens in the real instrument situation. Corrosion in metallic systems is generally caused by a combination of redox potentials between different metal components, loss of the passivation layer and contamination of the metal surface with tiny metal fragments derived from the machining process. The beaker experiment you did doesn't adequately test the real situation in the instrument. Do the following: swage a ferrule onto a stainless steel tube and then store it in HCl. In most cases, you'll see corrosion occurring, especially at contact points where the swaging component intersects the surface of the ferrule even though the individual components won't necessarily show any sign of corrosion if tested alone. Secondly, the passivated layer on the surface of stainless steel is not completely inert to cycles of exposure to acidic and basic solutions. If you alternate exposure of the stainless steel component to alternating extremes of pH, you will also generally observe corrosion with corrosive eluents. For this reason, it's generally inadvisable to use corrosive reagents such as HCl without taking extreme care to avoid corrosion problems. If you really find it necessary to use stainless steel in conjunction with HCl, it's highly advisable that you frequently passivate your instrument, avoid eluents containing chelating agents and high pH eluents. A better choice, though, is to use a noncorrosive acid such as methanesulfonic acid.

[HW Mueller]

Well, maybe I oversimplified tests a bit, what I use is a piece of the piping, from my system, polished in one place, the rest having flaws and rough cut edges. In short, I have never seen any corrosion, tarnishing, etc. anywhere, in connection with Cl- (max. 0.15M) at a pH of 7 or higher (years of contact). I don´t recall what acidic Cl- conc. I tested, but any of the HCl solutions I tried failed the test (turned black).
So I wonder, Ron, do you see any signs of oxidation or corrosion on the visible parts of your HPLC after having run your HCl?
Chris, I wonder whether the corrosion you mention was seen even with something like 4mM HCl. Actually an answer is not so important as I agree that it´s prudent to avoid acidic Cl-.